1. Field of the Invention
This invention relates generally to a fuel injector for an engine and, more particularly, to a fuel injector for a locomotive diesel engine, where the fuel injector employs a dead coil spring to reduce spring wear and reduce combustion emissions.
2. Discussion of the Related Art
Certain engines, such as the EMD 710 locomotive diesel engine, employ fuel injectors that inject a controlled amount of fuel into the cylinders of the engine. FIG. 1 is a representative example of a fuel injector assembly 10 used for this purpose. The assembly 10 is mounted to a cylinder head 14, such as a cylinder head for a diesel engine. The assembly 10 includes a fuel injector 12 positioned within the cylinder head 14 so that a spray tip 16 of the fuel injector 12 extends into an engine cylinder (not shown). Each cylinder of the several cylinders in the engine would include such a fuel injector assembly. The fuel injector 12 is secured to the cylinder head 14 by a clamp 18. A rocker arm 20 in combination with a spring 22 mounted on a shaft 24 controls the fuel injected into the cylinder in a manner that is well understood in the art.
Known fuel injectors employ a valve needle slidably positioned within a nozzle body of the fuel injector 12. A spring biases the needle to close the spray tip 16 and fuel pressure from the fuel applied to the fuel injector 12 moves the needle against the bias of the spring to inject the fuel into the cylinder through the spray tip 16. The fuel pressure required to move the needle is determined by the geometry of the needle and the force generated by the spring. This pressure is referred to as the valve opening pressure (VOP).
The standard spring used in a fuel injector for this purpose has a high wear rate at the end coils of the spring as a result of the repetitive opening and closing of the spray tip 16. Particularly, tangs at the ends of the spring wear into a first adjacent coil of the spring. This spring wear results in significant VOP loss over time, and decreases the spring force and reduces the ability of the spring to close the spray tip 16.
After a certain amount of spring wear, combustion gases from the cylinder can blow back into the nozzle body and throughout the internal passageways of the injector, which leads to various emissions from the fuel injector 12, such as smoke and/or mechanical failure. Also, a reduction in the spring force may prevent the needle from completing closing the spray tip 16, which results in fuel dripping into the cylinder that causes injector “gum up.” Further, because of the wear over the life of the spring, the fuel injector 12 requires a high VOP set point to offset the high VOP loss over time. This higher VOP set point produces higher levels of NOx emissions, which are detrimental to the environment. The higher NOx emissions makes it more difficult to meet U.S. EPA Tier 1 locomotive emission standards.